The present invention is directed to an orthopedic implant. More specifically, the orthopedic implant is suitable for arthroplasty procedures where optimized multifunctional behavior of the implant is desired. The implant may include the ability for load sharing between the implant and host bone, and the restoration of motion, such as in the replacement of a spinal disc.
Orthopedic implants have been used in the past to repair damage to the skeleton and related structures, and to restore mobility and function. For example, various devices, such as pins, rods, surgical mesh and screws, have been used to join fractured bones in the proper orientation for repair.
Implants that restore the function to a damaged joint have also been used. Surgery intended to restore function to a joint is referred to as arthroplasty. A successful arthroplasty may eliminate pain and prevent the degradation of adjacent tissue. Arthroplasty has been performed on knees, hips and shoulders by replacing portions of the joint with implants. Presently available implants for arthroplasty may result in stress shielding, meaning that the stress normally felt by bone adjacent to the implant is reduced due to the stiffness of the implant. When a bone is stress shielded, it typically reduces in size and strength according to Wolf""s Law, increasing the chance of breakage.
In some instances, instead of replacing a damaged joint, the joint is merely fused in a single position. Surgery intended to fuse a joint rather than to restore mobility is referred to as arthrodesis. Arthrodesis is particularly common for the complex load-bearing joints of the spine. Spinal fusion may be performed to remedy failure of a spinal disc. Spinal discs perform spacing, articulation, and cushioning functions between the vertebrae on either side of the disc. If the normal properties of a disc are compromised, these functions can be seriously reduced. Disc collapse or narrowing reduces the space between vertebrae, and damage to the disc can cause it to bulge or rupture, possibly extruding into the spinal canal or neural foramen. These changes can cause debilitating back and distal pain, numbness, or weakness.
Orthopedic implants may be used in arthrodesis to stabilize the spine and promote fusion. The two main approaches to implant-aided spinal fusion are anterior and posterior. Anterior fusion techniques are widely used, primarily due to the recent appearance of Interbody Fusion Devices (IBFDs). IBFDs are inserted from an anterior approach into the space normally occupied by the disc for space retention, stabilization and load bearing. Posterior fusion is accomplished by cutting through the musculature of the back, exposing the spinal segments, and fixing adjacent vertebra using hardware typically consisting of metal rods, screws, and other devices. Bone harvested from the patient""s iliac crest (autograft), donor bone (allograft), or other synthetic biocompatible material is sometimes also packed into the space to induce fusion.
U.S. Pat. No. 5,860,973 (Michelson) discloses an implant which is placed translaterally between two discs. The implant, which is typically installed as a pair of implants, is cylindrical and is filled with fusion promoting material. During the installation, holes are bored between the vertebra and the implant is placed within the holes. The vertebra then grow toward one another and fuse together.
Another way to treat spinal damage is to replace the damaged vertebra or disc with some form of spacer. For example U.S. Pat. No. 5,702,451 (Biedermann) discloses a space holder for a vertebra or spinal disc consisting of a hollow sleeve perforated with diamond-shaped holes. The holes are sized and arranged such that when different lengths of sleeve are cut, the recesses along the edge of the cut resulting from the diamond shaped holes are uniform and able to be mated with projections on an end cap.
Both spinal fusion, such as disclosed by Michelson, and the use of spacers, such as disclosed by Biedermann, limit the mobility of the spine by fixing two adjacent vertebra relative to one another. In addition to reduced mobility, these arrangements do not compensate for the shock absorption lost when a disc is damaged or removed.
Attempts to restore lost function to damaged spinal joints (arthroplasty) have also been made. For example, replacement of entire discs or simply the nucleus pulposis (center portion of the disc) have been proposed. Some attempts use elastomers to mimic the shock absorption and flexibility of the natural disc. However, the complex load bearing behavior of a disc has not been successfully reproduced with an elastomer, and such implants are prone to wear and failure. For example, U.S. Pat. No. 5,674,294 (Bainville) describes an intervertebral disc spacer having two metal half-envelopes which confine between them a cushion. Similarly, implants using various liquids and gels have also been attempted. These implants are subject to failure by rupture or drying out, just like a disc. Mechanical approaches to disc replacement have also been attempted. For example, articulating surfaces and spring-based structures have been proposed. In addition to failing to accurately perform the functions of the replaced disc, these structures are multi-component and particles due to wear of articulating components can result in adverse biological responses or increase the possibility of mechanical failure. For example, U.S. Pat. No. 5,893,889 (Harrington) describes an artificial disc having upper and lower members joined by a pivot ball and having a shock absorbing members fitted between the upper and lower member.
Total hip arthroplastics that use rigid stems as the load sharing devices between the femur and the acetabulum have been observed to experience the phenomena referred to as stress shielding also. In this case, the method of load transfer has been changed with the insertion of the implant. In the normal femur, the loads are applied to the femoral head and transferred along the length of the femur through the cortical shell of the femur. In the case of the femur with an implant, the loads are applied to the prosthesis which transfers the loads distally down the prosthesis and gradually transfer the loads from the prosthesis to the inside of the cortical shell. This results in a significant portion of the proximal portion of the femur no longer experiencing a normal stress condition. This condition will then result in a loss of bone mass surrounding the distal portion of the device. Consequences of this bone loss include loss of support for the device, which will allow the device to move and become painful, and, should revision of the device required, then there may be insufficient bone for support of the subsequent implant.
A number of approaches have been attempted to solve this problem. These include use of composite materials for controlled stiffness of the bulk material, modifications of the cross section of the device to reduce stiffness (this includes local reduction in cross section and hollow stems) and incorporation of slits in the device to increase flexibility. None of these approaches have been successful in that the compromises required to achieve the reduction in stiffness did not find the proper compromise between the required strengths and stiffness.
In total knee arthroplastics wear surfaces are typically made up of two materials, a polymer and a metal. Typically, ultra high molecular weight polyethylene (UHMWPE) is used as the polymer. While this material has excellent wear properties, it is not a wear free surface. The cartilage of the normal knee is capable of producing a fluid film upon the application of mechanical stresses to it. This fluid film is then used as a lubricant to reduce the coefficient of friction between the two cartilage wear surfaces. There is no fluid film lubricant in the total knee joint implants presently known. Instead the materials articulate directly on one other resulting in the generation of wear debris and possibly adverse biological responses.
Several attempts have been made to incorporate stochastic foam materials to reproduce this fluid film lubrication mechanism in the knee joint. However none of these approaches have been successful in reproducing the functionally graded material properties required for this application.
Accordingly, one embodiment of the present invention is directed to an orthopedic implant including a first plate, a second plate, an axial support between the first plate and the second plate, and one or more torsional supports connecting the first plate and the second plate. The torsional supports, the first plate and the second plate may be integrally formed.